Medical terms, defined.

C3b is the major cleavage fragment of complement C3 and the key opsonin of the complement system. It coats pathogens to enhance phagocytosis (via CR1 on phagocytes), forms C3 and C5 convertases that amplify the cascade, and initiates the...

MAC is a high-yield abbreviation with several meanings on boards: (1) Minimum Alveolar Concentration, the potency measure of inhaled anesthetics, (2) Membrane Attack Complex (C5b-9) of complement, (3) Mycobacterium avium complex, an oppo...

The alternative pathway is an antibody-independent arm of the complement system (innate immunity) that is spontaneously and continuously activated at a low level by hydrolysis of C3 to C3(H2O). It is amplified on microbial surfaces lacki...

Anaphylatoxins are small complement cleavage peptides (C3a, C4a, C5a) that trigger mast cell/basophil degranulation, vasodilation, increased vascular permeability, and smooth muscle contraction. C5a is the most potent and is also a power...

Human leukocyte antigens (HLA) are the human version of the major histocompatibility complex (MHC), encoded on chromosome 6p. MHC class I (HLA-A, B, C) presents endogenous antigens to CD8+ T cells; MHC class II (HLA-DP, DQ, DR) presents...

Central tolerance is the process by which developing self-reactive T and B lymphocytes are eliminated or edited in primary lymphoid organs (thymus for T cells, bone marrow for B cells). Self-reactive clones undergo negative selection (ap...

Positive selection is the thymic process in the cortex where immature CD4+CD8+ (double-positive) thymocytes whose TCRs bind self-MHC on cortical epithelial cells with low-to-moderate affinity are rescued from apoptosis ("death by neglect...

Clonal selection is the principle of adaptive immunity in which an antigen selectively binds to and activates only those pre-existing lymphocytes (B or T cells) whose receptors are specific for it, driving their proliferation (clonal exp...

Thymic selection is the process by which developing T cells (thymocytes) are screened in the thymus so that only cells that recognize self-MHC (positive selection, cortex) and do not react strongly to self-antigens (negative selection, m...

Peripheral tolerance is the set of mechanisms that silence self-reactive T and B cells that escaped central tolerance (thymus/bone marrow) once they are in the periphery (lymph nodes, spleen, tissues). Key mechanisms are anergy, regulato...

IgD is an immunoglobulin isotype expressed mainly as a membrane-bound B-cell receptor on mature naive B cells, co-expressed with IgM via alternative splicing of a single heavy-chain mRNA transcript. It is present in only trace amounts in...

IgM is the first antibody produced in a primary immune response and exists as a pentamer (in serum) with a J chain, making it the largest immunoglobulin and the most efficient at fixing complement. It does not cross the placenta, so elev...

IgE is the least abundant immunoglobulin isotype, bound with high affinity (FcεRI) to mast cells and basophils, and mediates type I (immediate) hypersensitivity and defense against helminths. Cross-linking of surface-bound IgE by allerge...

IgA is the main antibody class of mucosal immunity, found as secretory dimeric IgA in tears, saliva, breast milk, and GI/respiratory/genitourinary secretions. It prevents pathogen adherence to mucosa (does not fix complement or opsonize...

Coagulase-positive describes bacteria that produce coagulase, an enzyme that converts fibrinogen to fibrin and clots plasma. In boards, it almost always means Staphylococcus aureus, which separates it from the coagulase-negative staphylo...

Coagulase-negative describes staphylococci that do not produce coagulase, so they fail to clot plasma. The key organisms are S. epidermidis (novobiocin-sensitive; prosthetic device/catheter infections) and S. saprophyticus (novobiocin-re...

Catalase-positive organisms produce catalase, an enzyme that converts hydrogen peroxide (H2O2) into water and oxygen (2 H2O2 → 2 H2O + O2), seen as bubbling when a colony is mixed with H2O2. Catalase positivity separates Staphylococcus (...

Complement fixation is the consumption (binding) of complement by antigen-antibody complexes. The complement fixation test (CFT) exploits this to detect specific antibody or antigen in serum: if complement is "fixed" by patient antibody-...

Catalase-negative organisms lack the enzyme catalase, so they cannot convert hydrogen peroxide (H2O2) into water and oxygen. On a slide test, adding H2O2 produces no bubbles. The main catalase-negative bacteria are Streptococcus and Ente...

Delayed-type hypersensitivity (DTH) is a Type IV hypersensitivity reaction mediated by sensitized T cells (not antibody), appearing 48–72 hours after antigen re-exposure. CD4+ Th1 cells release IFN-γ to activate macrophages (e.g., TB/PPD...

Oxidase-positive describes bacteria that produce cytochrome c oxidase, part of the electron transport chain. These organisms turn an oxidase reagent (tetramethyl-p-phenylenediamine) dark blue/purple. High-yield oxidase-positive gram-nega...

An aerobe is an organism that uses oxygen for growth and metabolism, generating ATP via aerobic respiration with O2 as the terminal electron acceptor. Obligate aerobes require O2; facultative anaerobes can switch to fermentation or anaer...

Oxidase-negative describes bacteria lacking cytochrome c oxidase, so they do not turn the oxidase reagent (tetramethyl-p-phenylenediamine) purple. Nearly all Enterobacterales (E. coli, Klebsiella, Salmonella, Shigella, Proteus, Enterobac...

Acid-fast describes organisms whose cell walls contain long-chain mycolic acids, so they resist decolorization by acid-alcohol after staining with carbol fuchsin (Ziehl-Neelsen/Kinyoun). They stain red against a blue background. Classic...

Teichoic acids are anionic polymers of glycerol phosphate or ribitol phosphate found only in the cell walls of gram-positive bacteria. They are either wall teichoic acids (linked to peptidoglycan) or lipoteichoic acids (anchored to the c...

The Fab (fragment, antigen-binding) region is the arm of an antibody (Y-shaped immunoglobulin) that binds antigen. It consists of the entire light chain (VL + CL) plus the VH and CH1 domains of the heavy chain, and contains the hypervari...

Spore-forming bacteria produce highly resistant, dormant endospores under nutrient-poor conditions. The medically important spore formers are gram-positive rods: Bacillus (B. anthracis, B. cereus) and Clostridium (C. tetani, C. botulinum...

Lipoteichoic acid (LTA) is an amphipathic cell wall glycolipid unique to gram-positive bacteria. It is anchored in the cytoplasmic membrane and extends through the peptidoglycan layer. It acts as a PAMP that binds TLR2 and triggers cytok...

LPS (lipopolysaccharide), also called endotoxin, is the major outer membrane component of gram-negative bacteria. Its lipid A moiety binds TLR4 (with CD14 and MD-2) on macrophages, triggering release of IL-1, IL-6, and TNF-α, which cause...

A flagellum is a long, whip-like locomotor appendage. In bacteria it is a rigid helical filament made of flagellin that rotates (powered by proton motive force) to propel the cell. In eukaryotes (e.g., sperm) it is a "9+2" microtubule st...

Lysogeny is the bacteriophage life cycle in which phage DNA integrates into the host bacterial chromosome as a prophage and replicates passively with the host without lysing the cell. Induction (e.g., by SOS response/UV) can switch it to...

The lytic cycle is a viral replication pathway in which a virus (or bacteriophage) infects a host cell, hijacks its machinery to replicate, and then lyses the cell to release progeny virions. It contrasts with the lysogenic cycle, in whi...

A transposon ("jumping gene") is a mobile DNA segment that can move from one genomic location to another, either by a cut-and-paste mechanism (DNA transposons, via transposase) or a copy-and-paste mechanism through an RNA intermediate (r...

A plasmid is a small, circular, double-stranded, extrachromosomal DNA molecule in bacteria that replicates independently of the chromosome. Plasmids often carry nonessential but advantageous genes, such as antibiotic resistance, virulenc...

ESBLs are plasmid-encoded enzymes produced mainly by Enterobacterales (E. coli, Klebsiella pneumoniae) that hydrolyze penicillins, cephalosporins (1st–4th generation), and aztreonam, but not cephamycins or carbapenems. They are inhibited...

Beta-lactamases are bacterial enzymes that hydrolyze the beta-lactam ring of penicillins, cephalosporins, and related drugs, rendering them inactive. They are a major mechanism of antibiotic resistance (e.g., Staphylococcus aureus, H. in...

Minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial that prevents visible growth of a microorganism in vitro after overnight incubation. It measures bacteriostatic potency (not killing) and is compared...

Wet gangrene is tissue necrosis complicated by superimposed bacterial infection, causing liquefactive necrosis with swelling, blistering, foul odor, and poor demarcation between healthy and dead tissue. It is a surgical emergency with a...

Dutcher bodies are intranuclear, PAS-positive pseudoinclusions seen in plasma cells, made of invaginations of cytoplasm containing immunoglobulin. They are classically associated with Waldenström macroglobulinemia (lymphoplasmacytic lymp...

MIC (Minimum Inhibitory Concentration) is the lowest concentration of an antimicrobial that prevents visible growth of a microorganism in vitro after overnight incubation. It guides antibiotic selection and dosing and is interpreted agai...

Minimum bactericidal concentration (MBC) is the lowest concentration of an antimicrobial that kills ≥99.9% (≥3-log10 reduction) of the initial bacterial inoculum after a fixed incubation period (typically 18–24 h). It is always ≥ the MIC...

Russell bodies are eosinophilic, homogeneous, round intracytoplasmic inclusions in plasma cells made of accumulated immunoglobulin (usually IgM or IgG) that is retained in the dilated rough endoplasmic reticulum. They signal excessive or...

"Owl eye" inclusion is a large intranuclear viral inclusion with a clear halo, producing a nucleus that looks like an owl's eye. It is the classic cytopathic effect of cytomegalovirus (CMV, HHV-5). It is seen in infected cells (enlarged...

Guarnieri bodies are cytoplasmic eosinophilic inclusion bodies found in epithelial cells infected with variola (smallpox) virus and vaccinia virus. They are sites of viral replication (viral factories), reflecting the unique feature of p...

Negri bodies are eosinophilic, cytoplasmic inclusions found in neurons infected with rabies virus (Rhabdoviridae). They are pathognomonic for rabies and classically seen in Purkinje cells of the cerebellum and pyramidal neurons of the hi...

Schaumann bodies are laminated, concentrically calcified protein-calcium inclusions found within the cytoplasm of multinucleated giant cells in granulomas, classically in sarcoidosis. They are not specific for sarcoidosis and can be seen...

Cowdry type A inclusions are eosinophilic intranuclear viral inclusion bodies surrounded by a clear halo with margination of chromatin. They are seen in herpesvirus infections (HSV-1/2, VZV) and classically also in CMV-related and yellow...

Asteroid bodies are star-shaped eosinophilic inclusions within multinucleated giant cells of non-caseating granulomas, classically seen in sarcoidosis. They are not specific to sarcoidosis, though, and also appear in other granulomatous...

Anitschkow cells (caterpillar cells) are modified, activated macrophages with chromatin condensed into a central, wavy ribbon-like bar that looks like a caterpillar on longitudinal section and an "owl-eye" on cross-section. They are foun...

Aschoff bodies are pathognomonic granulomatous lesions of acute rheumatic fever (ARF), found in the myocardium (and other heart layers). They consist of a central zone of fibrinoid necrosis surrounded by T lymphocytes, plasma cells, and...